535
Fig. 6. X-ray diffraction pattern for MoS
2
thin films deposited at different deposition times
For MoSe
2
thin films deposited at deposition time of 10 – 30 minutes, peaks belonging to MoS
2
are detected. However, it can be seen in the figure that at 10 minutes deposition time, only one peak identified as MoS
2
peak is present at 2θ = 50.1° while the peaks for the stainless steel are more visible. However, at 15 minutes deposition time, a MoS2
peak at 2θ = 44.4° start to emerge and become more distinct from the neighbouring peak indicating that the 104 plane films grow
with time. The stainless steel peak at 2θ = 43.6° which is most intense at 10 minutes have shrunk at deposition time 30 minutes due to the growth of 104 plane films which has surmount over the
stainless steel peaks. As for the 105 plane structure of MoS
2
at 2θ = 50.1° and 220 plane structure of stainless steel at 2θ = 74.7°, the deposition time does not have any influence on the
thin films, as they appear as constant peaks throughout.
Table 2: Comparison of experimental‘d’ values with JCPDS data for MoS
2
thin film on stainless steel substrate with different deposition time
3.3 Optical Characterization
The transition metal chalcogenides are usually indirect band gap semiconductors [23]. The optical absorption spectrum was taken for both MoSe
2
and MoS
2
thin film using an identical ITO- coated glass plate as reference. A graph of
αhv
2
vs. hv is drawn and the linear portion of the graph is extrapolated to the energy axis. The intercepts on the energy axis at
α=0 gives the band gaps of these compounds.
Fig. 7a shows the αhv
2
versus hv plots of MoSe
2
thin films deposited at different deposition times see inset graph. The variation of band gap energy E
g
with the deposition time shows non-linear decrease is presented Fig. 7b. It is observed that with the increase of deposition
time of the films, the E
g
decreased from 1.22 eV to 1.12 eV, in good E
g
range with the reported value [24].
Angle 2θ h k l Standard
Å Experimental Å
‘d’
JCPDS
10 min 15 min
20 min 25 min
30 min 43.58
1 1 1 2.0750
2.0757 2.0752
2.0735 2.0738
2.0714 44.37
1 0 4 2.0400
- 2.0345
2.0331 2.0383
2.0334 50.08
1 0 5 1.8200
1.7985 1.8027
1.8017 1.7961
1.8006 74.70
2 2 0 1.2697
1.2707 1.2708
1.2705 1.2693
1.2705
536 For the MoS
2
thin films, the αhv
2
versus hv plots of the deposited films at different deposition times is shown in Fig. 8a see inset graph. Fig. 8b presents the non-linear decrease
for the variation of band gap energy E
g
with different deposition time. The figure shows that the E
g
decreased from 1.74 eV to 1.64 eV with increase of deposition time for the MoS
2
thin films, in good range with the reported value [25].
Fig. 7. Variation of MoSe
2
thin films deposited at different deposition times. a αhv
2
vs. hv and b bandgap vs.deposition time
Fig. 8. Variation of MoS
2
thin films deposited at different deposition times. a αhv
2
vs. hv and b bandgap vs.deposition time
537
3.4 Morphological Characterization
The surface morphology of the MoSe
2
and MoS
2
thin films were determined by scanning electron microscope SEM. The SEM micrograph of the MoSe
2
and MoS
2
thin film deposited on indium-tin-oxide ITO glass substrates at different deposition time is shown in Figure 9a - 9c
and Figure 10a - 10c.
Fig. 9. Surface morphology of MoSe
2
thin film deposited at a 10 b 20 and c 30 minutes.
538
Fig. 10. Surface morphology of MoS
2
thin film deposited at a 10 b 20 and c 30 minutes
From SEM analysis, as deposition time increases, the surface of the MoSe
2
and MoS
2
thin films are clearly seen to be in layered structure. At deposition time 30 minutes, the surface of the
thin film is found to be the best with a uniform and continuous morphology.
4. Conclusion
